Pakistan's prime minister on Thursday demanded “immediate action” over the murder of a pregnant woman who was bludgeoned to death outside a courthouse, as her husband revealed in a grisly twist that he strangled his first wife.

Farzana Parveen was murdered on Tuesday outside the High Court in the eastern city of Lahore by more than two dozen brick-wielding attackers, including her brother and father, for marrying against her family's wishes - while police stood by.

The brazen, brutal nature of the killing, in broad daylight in the centre of Pakistan's second-largest city, has triggered outrage around the world.

The United States on Thursday branded the incident “heinous”, with State Department spokeswoman Jen Psaki denouncing “unjustifiable acts” against women around the world, especially “violence that occurs in the name of tradition and honour, such as so-called honour killings”.

The attack also casts a spotlight on Pakistan's controversial blood-money laws which allow relatives of homicide victims to forgive their perpetrators - who, in cases such as this, are often also family members.

In a startling twist, Parveen's husband Mohammad Iqbal, 45, admitted to AFP on Thursday that he had killed his first wife - and was spared prison because he was forgiven for the act by his son.

“I was in love with Farzana and killed my first wife because of this love,” Iqbal said, adding that he had strangled her.

After admitting to the murder he switched off his phone and did not respond to further calls.

Zulfiqar Hameed, a senior police officer investigating the killing of Parveen, said police would be filing a report to the government detailing Iqbal's past.

“Iqbal was a notorious character and he had murdered his first wife six years ago,” Hameed said.

“He was arrested and later released after a compromise with his family.”

Hundreds of women are murdered by relatives in Pakistan each year supposedly to defend family “honour”, but the fact that police officers guarding the court apparently did nothing to intervene to save 25-year-old Parveen has added to the outrage over the killing.

Prime Minister Nawaz Sharif has told the Punjab chief minister, his brother Shahbaz Sharif, to act over the “brutal killing of lady in the premises of high court in the presence of police”, a statement from his office said.

“I am directing the Chief Minister to take immediate action and report must be submitted by this evening to my office,” Sharif said in the statement.

“This crime is totally unacceptable and must be dealt with in accordance with law promptly.”

Parveen, who was three months pregnant, had gone to court to testify in Iqbal's defence after he was accused by her relatives of kidnapping her and forcing her into marriage.

Iqbal, a farmer, told AFP he had been receiving death threats from his in-laws, and said he did not believe police were actively pursuing his wife's killers.

“I am already upset and worried but now they are threatening to kill me as well,” he said.

He said Parveen's family had initially agreed to their marriage but later changed their mind after he did not pay them a big enough dowry.

And he shed further light on Parveen's horrifying last moments.

“Five to six people were hitting her in the head, she was shouting for help, she was screaming but they killed my helpless wife,” he said. “We were in love.”

The incident gained prompt attention from the global media and international human right activists.

The UN High Commissioner for Human Rights Navi Pillay strongly condemned the killing on Wednesday and urged Pakistan to take “urgent and strong measures” to put an end to so-called honour killings.

British Foreign Secretary William described the murder as “barbaric” and urged the Pakistani government to fully investigate it.

“I am shocked and appalled by the death of Farzana Parveen: both for the appalling manner of her death, and the unspeakable cruelty and injustice of murdering a woman for exercising her basic right to choose who to love and marry,” Hague said in a statement.

“There is absolutely no honour in honour killings and I urge the government of Pakistan to do all in its power to eradicate this barbaric practice.

Last year, 869 women died in so-called honour killings, according to the independent Human Rights Commission of Pakistan. - Sapa-AFP

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